You know the first rule of used gear, right? Sellers always ask more for their used gear than they'd be willing to pay you for yours. My own rule of thumb is to pay 75% of new if something is less than a year old and still the current model; 50% if it's more than a year old and a single generation out of date. I typically don't buy used beyond that. I suppose if I were going to buy something like a Radion gen 2 that's multiple years old and two generations removed i'd probably be thinking 25%. Unlikely that most sellers would view that as anything beyond 'low balling' thus my not bothering.